1. Field of the Invention
The present invention relates to a variable-optical-delay apparatus for delaying an optical signal by a delay time determined in accordance with an input optical signal.
2. Description of the Prior Art
Listed below are methods used to control the time-delay of an optical signal.
1) A method in which a plurality of optical paths of different lengths is prepared and the optical path to be used is selected according to the delay time required.
2) A method of controlling the delay time by shifting the optical signal frequency and propagating the signal along an optical path having frequency dispersion characteristics.
3) A method of controlling the delay time by shifting the optical signal frequency and using a reflector having frequency dispersion characteristics, such as a filter, to reflect the optical path.
4) A method in which the optical signal is converted to an electric signal, and the electric signal is delayed and then converted back to an optical signal.
5) A method that uses what is called a cyclic type optical packet buffer in which a switch positioned between the optical input and output sections is used to connect an optical fiber loop delay apparatus that includes an optical amplifier.
Japanese Patent Laid-Open Publication No 2001-209082 discloses an optical packet buffer, shown in FIG. 9, that is similar to the present invention. The object of the disclosed invention is to facilitate size-reduction and integration, eliminate instability caused by oscillation and provide the ability to handle variable-length packets, and is therefore not constrained by packet length. In this optical packet buffer, optical signals input via an input waveguide are converted to a fixed wavelength by a variable-wavelength conversion circuit and input to the delay loop via an optical coupler. Each time the signals circulate within the delay loop, a wavelength shifter shifts the signal wavelength by a set amount. The signals thus are passed through a specific wavelength extraction circuit, which extracts signals of a specific wavelength and outputs those signals via an output waveguide.
Thus, optical packets input via the input waveguide are converted to a prescribed wavelength by the variable-wavelength conversion circuit and input to the delay loop via an optical coupler. The optical packet buffer has a wavelength shifter that shifts the wavelength of the signals. Signals having a wavelength that is not specified are passed through the loop from port a to port b, while signals having a specific wavelength are output through port c by the specific wavelength extraction circuit disposed between ports a and c. The number of cycles (the number of times the optical signal packets pass the wavelength shifter) undergone by the packets from input to separation by the specific wavelength extraction circuit, is determined by the packet length set by the variable-wavelength conversion circuit, thereby determining the packet delay time.
As described, the variable-wavelength conversion circuit is used to set the delay time of the above optical packet buffer by setting the wavelength of the optical packets prior to the entry of the packets into the delay loop. Thus, a variable-wavelength conversion circuit and a wavelength shifter are used to change the wavelength of the optical packets.